The discussion below is merely provided for general technological background information and is not intended to be used as an aid in determining the scope of the claimed subject matter.
To produce granules of a liquid solution or melt, such as an aqueous or non-aqueous ammonium nitrate or urea solution, said solution or melt is sprayed into a granulation compartment containing a fluidized bed of solid nuclei. The fluidized bed is fluidized by introducing a fluidization gas, usually air, through the bed of nuclei. The nuclei grow by accretion, i.e. solidification and crystallization of the sprayed urea liquid on them, to form granules of a desired average size, which are subsequently withdrawn from the fluidized bed reactor, also called a granulation reactor or granulator, wordings which are used interchangeably in this patent document.
The fluidization air is generally blown into the granulation compartments by means of blowers. In a subsequent compartment, the air is stripped from fine solid material, e.g., in a scrubber, a cyclone or a similar separator. The air is usually removed from the granulation reactor by means of an exhaust fan.
An example of such reactor for the production of urea granules is disclosed in U.S. Pat. No. 3,533,829 (Azote et Produits Chimiques S.A., 1970), which discloses a fluidized bed reactor comprising at least one granulation compartment with at least one air inlet and which comprises two air moving devices: (i) a blower upstream of the fluidized bed configured to push fluidizing air through said at least one air inlet into the granulation compartment, and (ii) an exhaust fan, downstream of the granulation compartment, but not configured to draw air through the at least one air inlet into the granulation compartment; the exhaust fan merely serves the exhaustion of air leaving the system.
Also GB2046121 (MTA Müszaki Kémiai, 1980) discloses a fluidized bed reactor comprising at least one granulation compartment with at least one air inlet and which comprises two air moving devices: (i) a blower upstream of the fluidized bed configured to push fluidizing air through said at least one air inlet into the granulation compartment, and (ii) an exhaust fan, downstream of the granulation compartment, configured to remove air leaving the system, but not configured to draw air through the at least one air inlet into the granulation compartment; the exhaust fan merely serves the exhaustion of air leaving the wet scrubber, which is positioned after the fluidized bed reactor.
The produced granules are generally moved from the granulation compartment to an after-cooler, which can be integrated with the fluidized bed reactor. In the after-cooler, further dust is produced. Therefore, also air from the after-cooler is usually first treated in a scrubber before it can be vented.
To avoid leakage of air from the reactor, a slight underpressure, typically of about 0.1 to 10 mbar, preferably about 0.1 to 7 mbar, is created in the fluidized bed reactor compartment and/or after-cooler, as for example is disclosed in U.S. Pat. No. 5,779,945 (DSM N.V., 1998) and EP 2253374 A1 (Stamicarbon, 2010).